Mac OS X: easiest (free, non-QuickTime Pro) application for converting numbered folder of images to movie

The ffmpeg command line tool does this:

ffmpeg -n -i "image_%03d.jpg" output.m4v

ffmpeg will, in this example, look for files matching the pattern image_xxx.jpg in the current directory where the xxx part is a zero-padded decimal number, like 003. ffmpeg automatically chooses the output format based on the suffix of the output file, e.g., m4v.

Change -n to -y if you want ffmpeg to always overwrite output file without asking (useful for repeated tests).

I suggest you download ffmpeg from either homebrew or macports.

There is a freeware AppleScript-based application by the name of Sequimago which should fulfill your requirements admirably. It is available from this page at MacScripter, a forum site where Martin Michel, Sequimago's author, is an administrator.

This is his description of the program:

Requirements
Mac OS X 10.5 Leopard • QuickTime 7.2.1

Installation & Usage:
Download and extract the ZIP archive. Then open the script with a double click or drop a bunch of image files onto its icon. The script will then ask you to specify a file name and location for the new QuickTime movie. After you provided certain settings for your image sequence (e.g. frames per second/seconds per frame), your image sequence will be produced.

Supported Image Formats:
jpg, jpeg, gif, png, tiff, tif, psd, pict, bmp, pdf (only the first page of a PDF document is recognized)

Notes:
Sequimago currently uses the JPEG image format for the image sequence, which results in a smaller file size of the QuickTime movie. But you can easily edit the Python script to use alternative image formats (e.g. TIFF). The Python script is located at: Sequimago.app/Contents/Resources/crtimgseq.py

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P.S. -- I think you'll appreciate a comment he made in the course of explaining how he came to write the program:

I knew that you could easily create custom slide shows with QuickTime Pro, but why spent hard-earned $29.99 when you can get the same functionality free of charge by using built-in Mac OS X 10.5 technologies?

ImageMagick (available from macports) can do that, try running the command

convert -delay 30 'pic-*.png' movie.m4v

where 30 is the hundreths of seconds between frames and the files follow the pattern:

pic-001.png pic-002.png ...