A few options were suggested, depending on how automated or hands-on you want the process to be.

• Google Picasa was recommended as a simple starting point. It can automatically build a collage from selected photos using Create > Picture Collage. It’s fairly basic, but useful if you want something quick and easy.
• GIMP can be effective for building large collages, especially if you want more manual control. The tradeoff is that it can be time-consuming, and large collages may need a lot of memory.
• Lightroom 3+ was noted for a montage-like feature in the Print module, where you can position images on a page. However, it may be better suited to print layout than creating a standalone composite image.
• If you specifically mean a photo mosaic rather than a standard collage, that’s usually a separate category and may require dedicated mosaic software.

So, for quick automatic collages, start with Picasa; for more manual control, try GIMP; and for print-oriented page layouts, Lightroom may help.