Noteworthy SummonsI was originally avoiding talking about specific summons, since it begins to make the guide favor one style of caster over another. I already felt guilty about not giving psions any love. But, there seems to be some demand for it, so here goes.
Unicorn: This is a nice little mini-cleric, allowing you to cast a bunch of nice little healing spells when you need them. Appears on the Summon Nature's Ally IV list, and is added to the Summon Monster IV list by the Wild Soul prestige class.
Earth Elementals: As stated earlier, these guys can ignore walls, allowing you to find out what is going on behind that door without going there yourself. Requires the ability to speak Terran, or some form of Telepathy, or Chain of Eyes. This can also be used to search for hidden passages, pit traps, and to act as a nice surprise for the enemy. (Summon some Earth elementals, get them to earth glide to underneath the enemy, and pop out at the beginning of the fight.) Earth Elementals appear on the Summon Nature's Ally, and Summon Monster spell lists, at various levels. Thanks to Jason_Synder for the additional ideas on how to use them.
Creatures with large numbers of attacks: Jason_Synder suggests the use of these against Mirror Image using opponents. They make good summons anyway.
Fire Elementals: Allows you to attack wooden structures. Thanks to Jason_Synder for this idea. Fire Elementals appear on the Summon Nature's Ally and Summon Monster lists.
Pixies: Repeated, although weak, dispel magics. Best used on an opponents items, or against low-level casters. Appears on the Summon Nature's Ally VI list. Soltares puts forward the following advice: (Bold added by me)
Thoqqua particularly rock in any sort of underground adventure. Doors, walls, etc. become meaningless as your lava beasties burn a new tunnel straight to your objective. Want to modify terrain in a hurry (to make traps, escape chutes, prepare a massive cave-in?), they're the ones for the job. Plus they do some nice damage for their level, if you want to just throw them at mobs.
The Thoqqua appears on the Summon Nature's Ally III list. These guys speak common, so you don't need to worry about knowing their language. (Anything that doesn't have a language specified, and has an intelligence of 3 or greater knows common by default)
BenSan has been kind enough to go through all the books that list monsters, and provide this list. It is rather long, and he has mentioned that he will possibly go through and cut all the stuff out that is irrelevant at a later date
I gave all my sourcebooks a quick skim looking for monsters that are explicitly tagged as summonable with Summon Monster. Since I was looking mainly for things which bear further examination for spell-likes, I did not note animals which can be Fiendish/Celestialed (they only appear in PHB and Stormwrack, anyway) and since I was doing it for a binder I didn't take notes for Summon Nature's Ally. So, here it is. [sblock] PHB: SM2: Lemure (devil) SM3: Small elemental, Hell hound, dretch (demon) SM4: Lantern archon, mephit (any), yeth hound, howler SM5: Hound archon, medium elemental, achaierai, bearded devil, shadow mastiff SM6: Bralani eladrin, large elemental, janni, chaos beast, chain devil, xill SM7: Avoral guardinal, djinni, huge elemental, invisible stalker, red slaad, bone devil, babau demon SM8: Lillend, greater elemental, blue slaad, hellcat, vrock SM9: Couatl, leonal, elder elemental, green slaad, barbed devil, night hag, bebilith (demon), hezrou (demon) BoED: SMIII: Bariaur, Coure Eladrin, Musteval Guardinal SMIV: Cervidal Guardinal, Rhek SMV: Leskylor SMVI: Equinal SMVII: Asura, three-headed leskylor SMVIII: Hollyphant, lupinal guardinal, warden archon SMIX: Firre eladrin, moon dog, owl archon, quesar, sword archon, ursinal guardinal MM3: SMIV: Small storm elemental SMVI: Medium storm elemental SMVII: Arrow demon (replaces babau), large storm elemental SMVIII: Huge storm elemental SMIX: Greater storm elemental MM4: SMII: Clockwork Mender, Howler Wasp SMIII: Windrazor SMIV: Wrackspawn, Voor (yugoloth) SMV: Dwarf Ancestor SMVI: Inferno Spider, Corruptor of Fate (yugoloth) SMVII: Justice Archon Complete Arcane: SMIV: Elemental grue Fiend Folio: SMII: Kaorti, varoot (nerra) SMIII: Bacchae SMIV: Aoa droplet, Skulvyn (demon), bloodbag imp, euphoric imp, filth imp, kalareem (nerra), skeroloth (yugoloth) SMVI: Winged warrior formian. sillit (nerra), mud slaad SMVII: Movanic deva, armadon formian, ferrumach (rilmani) SMVIII: Maelephant, piscoloth (yugoloth) SMIX: Farastu (demodand), Wastrilith (demon), monadic deva, observer formian, cuprilach (rilmani) Ghostwalk: SMVI: Artaaglith (demon) [5th-level cleric casting, spell-like Animate Dead] Magic of Incarnum: SMVII: Souleater (replaces huge fiendish scorpion) Planar Handbook SMI: Elysian thrush (replaces celestial owl or monkey) SMII: Ur'Epona (replaces celestial riding dog) SMIV: Lesser nightmare (replaces fiendish dire wolf or hoowler) SMV: Energon (replaces medium elemental) SMVI: Elementite swarm (replaces large elemental) SMVII: Pack fiend (replaces Huge fiendish scorpion) SMVIII: Elsewhale (replaces celestial cachalot whale), Gaspar (replaces celestial triceratops or Colossal fiendish centipede), Limbo stalker (replaces blue slaad) SMIX: Unraveler (replaces green slaad) Anarchic, axiomatic, entropic, vivacious can replace celestial/fiendish Sandstorm: None specifically named, see below regarding: SMIV: Glass mephit, sulfur mephit Stormwrack: Several new animals to template SMVIII: Caller from the deeps (replaces greater elemental), echinoloth (yugoloth) (replaces blue slaad) [/sblock] Two special notes bear mentioning. First is that the PHB lists "mephit (any)", so while there are no new summons explicitly granted in Sandstorm, the new mephits may be summonable depending on DM. The other is that formians and imps were summonable in 3.0 but were removed from the list in revision. The result is that technically the odd variant formians and imps in Fiend Folio are summonable (because they are marked as such in their entries and were never reprinted without it) but normal formians and imps are not (because the only tag for them as such was in now-superceded 3.0 text.) Individual DMs may reasonably feel differently about this. On a personal note, I miss having formians on the list; as a result of their removal there are no core summonable LN monsters. It's my intention to give these a more leisurely trawling and mark up unusual or valuable abilities on these creatures, but I can't promise when.
Treantmonklvl20 points out that summoning a fiendish centipedes generally have more HD than other creatures of the same level. This is exceptionally handy with polymorph, allowing you to get, for example, a Bebelith with a 6th level spell and a 4th level spell. If you want to do it in the same round, you will need either Quicken Spell and an 8th level slot, or the Master Specialists 10th level ability.