The open source ImageJimage processing application and library can open a sequence of images (File -> Import -> Image Sequence), create an internal representation of the sequence (a so-called "stack"), and then save the stack as an AVI (File -> Save As -> AVI). It also has lots of I/O plugins, one of which may be able to export MPEG.
Due to all images having to be in memory before the AVI is created, you may be limited by main memory as to the number of images it can process.
Joined: Mar 22, 2005
Just noticed that I didn't mention this before, but ImageJ is completely automatable -either by using its internal macro language and recorder, or by using it as a Java library- so if you were looking for a programmatic way to do this, that's possible too.