Checkpoint from Thomson Reuters contains primary tax resources plus extensive secondary sources and expert analysis, including editorial content from the Research Institute of America (RIA), the authoritative Warren, Gorhman, & Lamont (WG&L) Treatises, and in-depth analysis of tax legislation. Checkpoint contains a robust full-text search functionality similar to Westlaw.
Bloomberg Law's Tax Practice Center combines primary and secondary tax resources, including the BNA Tax Portfolios. If you wish, you can set the Tax Practice Center as your homepage.
Lexis Advance Tax is a subsection of Lexis Advance entirely devoted to tax research. It contains primary and secondary sources for tax research, including Lexis Tax Advisor.
Westlaw's comprehensive collection of primary and secondary tax resources.
Intelliconnect contains primary and secondary sources for tax research, including the CCH Standard Federal Tax Reporter. It is possible to do full-text searching on Intelliconnect, but browsing the topics often works better.