If you're prone, you can still make special attacks, if you put up with the appropriate penalties. This includes:
Bull Rush
Yes, you can bull rush someone if you're prone. I mean, the RAW don't say otherwise, right? Your opponent gets a +4 to his AOO (unless you have the appropriate feat to avoid it), and you get no penalty to your opposed STR check. This can give your allies AOOs if you push your opponent through their threatened areas. If you choose not to follow him, your opponent leaves your threatened area, and you can take your move action to stand up safely.
Disarm
Not such a good idea, since you get a -4 and your opponent gets a +4 to the opposed attack roll. On the plus side, if it works, you can take your move action to stand up, and your opponent won't get an AOO, since he is unarmed.
Grapple
This one actually makes sense. +4 to your opponent's AOO (again, unless you have the right feat), and a -4 to your melee touch attack to grab him. No penalties to your opposed grapple checks.
Sunder
(See Disarm above)
Trip
(See Bull Rush above. Again, if it works, you're out of the threatened area.)
Also, you can cast spells that prevent an opponent from taking actions, such as ones that daze, stun, paralyze, etc., then stand afterwards. Casting on the defensive is no more difficult when you're prone than when you're standing.