wget

wget is very useful for acquiring data from, e.g., IRSA, the NASA Infrared Science Archive.

wget -nd -r -l1 -A*g09*_b4_20.fits http://irsa.ipac.caltech.edu/data/IGA/images/

The important elements:
-nd: don't reproduce the host directory structure
-r: recursive. Grab the files referred to by the page, not just the page itself
-l#: number of recursion levels
-A: "accept" wildcard