I guess it would depend on how you define a "mod". But if you consider a 'mod' to be everything you need to make a mission with completely new objects/units/vehicles/etc, then there are quite a few:
BAS Tonal Island / Units
An extremely good mod set on a fictional African island. Includes prolly the best looking island and units for OFP. Also includes a campaign and a couple missions, unlike modern addons....
Main downloadPatch #1Patch #2Special forces packSEBNam PackA vietnam war mod. Old and not the prettiest compared to newer addons, but the only complete 'Nam mod out there. It lacks vehicles, however (except helos). Doesn't come with any missions, but a ton of missions have been made using it.
LegaWarzAn interesting mod that pits Lego men against each other. I think it could be really cool, except (a) there is no custom island, and the lego units don't fit too well into the BIS ones; and (b) I don't think I've ever seen a mission made using this mod.
You can check this page
here for a few more mods, such as:
Battle over Hokkaido Japanese mod of great quality. Includes a campaign. It also includes some giant fighting robots that are tons of fun!
Invasion 1944WWII mod that has been in development since OFP's original release, I think. Oh yeah, and it isn't finished yet... and it includes no missions. But a ton of missions have been made using the demo addons, so I suppose it could be considered a mod...
And, as mentioned before, FDF and CSLA mods are great as well, although I've never played CSLA. Both mods come with missions and campaigns, however, so that makes them even better. Plus, a TON of missions have been made using FDF by other mission makers. Not so sure about CSLA.
There are also a number of other mods out there that I've seen mentioned breifly, but I'm not sure about their current status. Doing a search of the official forums should turn something up though:
Operation Northstar Canadian mod. Don't know who they are supposed to fight though...
Chinese mods There are a couple chinese/taiwanese mods out there. OFP Taiwan Workshop (OTW) is one, Republic of China (ROC) might be another....
That's about all I can think of. However, the only mods that I would consider 'complete' (having enough units and also missions/campaign) would be Tonal, CSLA and FDF, so they are worth downloading.