I also recommend breaking out your freshman algorithms and data structures text books. I favor Sahni and Knuth myself, but nearly anything that hurts to read should do.
Brad Wilson recommend the techInterview site.
"They have a bunch of logic puzzles (okay, well, mostly logic puzzles) that could be used in technical interviews. Solving the puzzles helps your thinking skills, and if you're interviewing someone, you may want to select from their list of questions (or modify it slightly based on a theme, etc.)."
Mable (not her real name) recommends these books:
"There are some good books to read, too, that you might recommend. K&R has a particularly clever solution to how-many-one-bits, and has, I believe, qsort and dirwalk. Plaugher's The Standard C Library is where I practiced my strings lib functions, and was good reading in general. Jon Bentley has written some books, including Programming Pearls and More Programming Pearls that are very readable and mildly appropriate to MS interviews. Lastly, there is Expert C Programming: Deep C Secrets, by Peter van der Linden, published by Sun Soft Press. It has an unforgettable orange cover with a blue fish on it, that is excellent reading overall, but in particular has a whole appendix with questions and answers for techie interviews."