A very popular chip is the UC3843. The only drawback is that you most likely need a current tranformer, since the current sense voltage is relatively high, about 0.7V.
A chip with much lower sense voltage (0.1V) is the MAX668, boost converter from Maxim. I cannot say for sure if you can use it for 100W output, but you can always use a MOSFET driver. Nice part. I used it in several designs.
I hope I understood correctly that you wanted a DC-DC 12V in, 24V out. If you can provide just a few more details, I may be able to suggest something.
Also check out Linear Technology, Texas Instruments, National, Micrel and ST websites.