Thursday: The Cooler, Duck Inn, Bada Bing's (earlier in the evening, at 6pm...not sure if this is every week)
Friday: Sher's Eldorado, Trackstirs
Saturday: Duck Inn, Lone Star
Sunday: Black Forest, Country Side, Downtown Lounge
Monday: Black Forest, Country Side, Jogger's, Rock 'N' Rodeo, Neighbors (not listed in the Eugene Weekly)
Tuesday: Country Side, Quackers, Taylors, The O Bar (although I had a bad experience there)
Wednesday: Charlie Mac's, Fathoms, Sher's Eldorado
I haven't been to all of these, but I most enjoyed the Black Forest, the Duck Inn and Neighbors (although I only karaoked there once). I've also heard the Downtown Lounge is good. As I said, I had a bad experience at The O Bar, although it had nothing to do with the DJ (he rocked). It had more to do with the bartender being rude to us. But that was several months ago, so who knows.
There's also a listing on the Eugene Weekly music page for Corvallis.
Go and enjoy!