isochore plots GC content over a sequence. It is intended for large sequences such as complete chromosomes or large genomic contigs, although interesting results can also be obtained from shorter sequences.
Here is a sample session with isochore. This reads yeast chromosome 3 from a remote web server, so the sequence reading may take a little time:
% isochore Input sequence: tgb:scchriii Output file [scchriii.iso]: click here for result