Komodo Edit is a free, multi-language editor with auto-complete, call tips, syntax highlighting, syntax checking and many other features. Available on Linux, Mac OS X and Windows.
Komodo IDE is Komodo Edit on steroids, with loads of additional features such as:
Open Komodo is the open-source codebase of Komodo Edit. The site features an SVN repository, code search tool, and a developer wiki maintained by ActiveState and members of the wider Komodo community.