Just to give you an idea of useful model packs, browse the website of www.geo-metricks.com. They offer a couple dozen different low-poly model collections. I don't work for them or get anything from them for referrals or anything. I've bought 3 collections from them though because they (IMHO) center on things that game creators ask for time and time again... vehicles, structures, and characters.
Since I don't have tons of money to spend on models, it also helps that a fair number of the models can be used in more than one genre. The WW II urban ruins in the "101 Historical Structures" collection easily work as WW 3 ruins or medieval ruins if I paint different textures on them. I think this would also be true of, say, a collection of bridges, ocean-going freighters, trees, animals, etc. etc.
For something a little more era-specific, I'd love to have a collection of late 19th-century sailing ships, a la the Horatio Hornblower or Alan Lewrie series. I'm probably going insane, but I think that a good naval warfare game is yet to be written, and I just don't have the modeling skills to make the ships.
