Also check out Vowpal Wabbit [0]. Although this is a tool that provides several online versions of classification, regression, etc, there's an online LDA module included as well. It takes advantage of the "hashing trick", meaning that you can build an LDA model with a fixed memory footprint. I've had a lot of success using this tool.

[0] https://github.com/JohnLangford/vowpal_wabbit