Spears edged out rapper 50 Cent, the US-based Cartoon Network and comeback singer Mariah Carey on Yahoo's American list of overall top searches.
The top news story searched for by Yahoo users was the Asian tsunami, which struck just before the end of 2004, followed by Iraq, and the Michael Jackson child sex trial.
Digital cameras, perennial favourite flowers, and the trendy iPod digital music player topped the list of most popular product searches on the site.
Over at rival Google, Janet Jackson emerged first on a list of news searches for 2005, surprisingly edging out Hurricane Katrina, which devastated the Gulf coast and New Orleans in August.
The Asian tsunami came next, followed by the XBox 360 portable games console ahead of the former Mr Jennifer Aniston, Brad Pitt, who saw his new belle Angelina Jolie come in ninth.
The iPod topped the list of the most popular items on the Froogle smart shopping site, followed by digital cameras.