I've got the Modern Player's Companions ... don't know as I've USED them that much. I've never found anything particularly broken about them, either. One of them DOES add a much-needed talent-tree to the Smart class ... but it's not a particularly stunning talent.
I'm still looking for a third starting talent for Smart. The class has alot going for it, but it seems like most of the time people take Savant "just to be taking it", and it actually becomes broken after a few levels ... but that's a grumble for another thread.
If you liked Alternity, you'll like the Menace Manual. It's got quite a few of the Dark*Matter creatures in there ... the Elohim, Sandmen, etc etc. It makes it much easier to run a Dark*Matter game, but you'll still need to have the D*M book for all the tastey tastey setting details. I've got that book in PDF and Hardcover, just because I use it so often. It would be wonderful if WotC would revisit that setting. Instead we get Urban Arcana.
Though over time Urban Arcana has grown on me. Sort of like mold. But if you've ever seen "The Life And Times of Juniper Lee" on Cartoon Network, you might understand how the setting can be cool. It's still D&D with the serial number filed off, but if you inject more real-world mythology and turn it up to 11 ... it's alot more fun than it seems at first blush.
--fje