Archivarius is very lightweight. It is under
constant development and gets better with each release. Two points to keep in mind:
1. It's
shareware2. Support for searching and manipulating Outlook PST files is there but the implementation is via OLE, which means that initial indexing takes over an hour (worth it though) and updating the index takes upwards of 10-15 minutes. A switch to COM (used in other indexers) is planned for a future release.
I love this app, having come to it as a longtime YDS/X1 user. Archivarius is obviously more expensive and is not as capable in terms of previewing content but it is infinitely faster, more configureable, and lighter on resources. Just to stress an earlier point, it's also actively developed. Also, I've found the developer to be extremely prompt in replying to bug reports and suggestions and to be very courteous.
YMMV.