There are not many lightweight rims available for 24" mountain bikes. The three options I know of are:
- Alex ACE20 - 507 x 17. 24, 28 and 32h exist, but 32h seem to be the only ones readily available. Weight is around 380-390g per rim (note: this rim may not be made any more);
- Velocity Aeroheat rim - about 420g. Probably stronger than the ACE20 but with additional weight.
- Alienation Deviant - 415g. Lighter than Aeroheat, heavier than ACE20, but not available with a v-brake compatible sidewall, so only for disk brake use;
- Sun Rims CR18 24x1.50 - 420g claimed, v-brake compatible;
- New!!! Stan's are now making a version of the Crest in 24" and it weighs a mere 310g (claimed with some actual examples at 307g and 308g). This is awesome news for anyone wanting to build a lightweight set of 24" XC wheels.
Only the ACE20 and Velocity Aeroheat rims will work with rim brakes (and obviously discs as well).
There are some lightweight BMX racing rims in a '24 inch' size, but these have an ERD of 520 to 540, whereas to fit standard 24" mountain bike tyres like the Schwalbe Moe Joes or Rocket Rons you will need an ERD of 507.
It seems that the ACE20 rim is no longer being made by Alex. It has disappeared from their website.
The Sun Rims CR18 24x1.50 also looks like it has been discontinued.
Stan's NoTubes are making a 24" version of the Crest; Awesome.