Three main options in my opinion.

If you fancy improving the original game then try distant stars ([MOD] Distant Stars - [04/02 Ent]0.9951 Released! - [08/03 Dip]0.7 Released). A huge mod which adds custom planets extra research and in one version an extra capital ship for each race (extra ships from a merge with Alliance hidden agendas).

If you fancy adding extra races to your came you should consider the Maelstrom mod  ([MOD] Maelstrom |Original v1.191|Entrenchment v1.051|Diplomacy v1.011) which adds three original (and highly playable) races to the game).

The other great race adding mod in my opinion is Stargate races (MOD: Stargate Races - 2010/09/13 - New Race Added) which in its current version adds the following races to the game: Ancients, Goa'uld, Wraith, Asgard and Ori. These won't mean anything to you if you don't know the Stargate series but are all great fun to play as. 

Which version of the game do you have? Obviously not all mods are available for all versions of the game. 

nice ad tactic syneptus lol,i would try the maelstrom mod coupled with totaleffects if i were you

on second thought maybe not because i forgot that maelstrom includes bailknights,oops

i thought the phase space effect was bailknight's (?.?)

okay then yea try maelsrom with totaleffect

lol

@wbino Do you have Entrenchment/Diplomacy? If not you can save five bucks if you buy them now... Most of the current mods are targetting the expansions.