ASM
http://asm.objectweb.org/
It is much faster than BCEL and supports generics and annotations.
One point about its architecture: in order to ensure high performance ASM is built around a parser that throws events (in contrast to BCEL where the parser builds a data structure). This is somewhat similar to the difference between SAX and DOM parsers. It takes some practice to get used to this kind of thinking.
EDIT (Following McDowell's comment): Indeed visitors are heavily used in ASM, but it's more than plain visitors: the visited data structure is lazily built by the parser, so if you're not interested in certain parts of the classfile (for example, you want to know the names of the methods but you don't care about their body), you can return a null from the visitMethod() method. This will make the parser skip the method body sections thereby preventing the (expensive) construction of the net of objects fully describing the method.
