I've always had trouble comming up with interesting basing ideas. By looking at other people's minis and thinking a lot I have come up with a number of different basing ideas I would like to share.
Classical GW style base
The classical GW base is either sand painted goblin green and drybrushed with yellow or a goblin green base with flock. I find the GW base quite boring.
Static Grass Base
Static Grass can add a lot to the classical style base. I prefer a dark brown or black base with bright green static grass.
Dried Leaves Base
Using oregano or other spices to make dried leaves gives a really nice autumn look .
Real rocks (surprisingly) make very realistic miniature rocks if they are painted grey and drybrushed.
I used Woodland Scenics Snow Effects for my snow bases. Mixing 2 parts snow with one part PVA glue and one part white paint gives a really nice and durable snow base.
For my water bases I use either nail polish like explained in this article or Woodland Scenics Water Effects. I apply the Water Effects and shape it, leave it to dry for 24 hours and apply a little dilluted ink.