San Francisco ? SideStep Inc., a popular alternative to larger online travel sites, is making its free price comparison service available through a Web browser.

Until last week, travelers had to install software on their computers to sift through a SideStep index of the best Internet deals from more than 100 airlines, hotels and rental car agencies.

The software threatened to lose its competitive edge as more industry upstarts introduce similar services accessible through Web browsers.

Since its November 2000 debut, SideStep has grown steadily.

The company says 7.7 million people have downloaded its software.

SideStep and similar services Mobissimo, Kayak, Farechase and Cheapflights hope to win customers from online travel giants Expedia Inc., Orbitz Inc. and Travelocity.com LP by offering a more comprehensive selection of discount deals.

Forrester Research estimates U.S. travelers spent about $53 billion on trips booked through the Internet last year. SideStep said it accounted for $500 million in trips.

To protect their turf, online travel agencies block some aggregators from listing their information. SideStep’s service, for instance, draws upon Orbitz’s data but can’t pull prices from Travelocity or Expedia.

SideStep also can’t list discount leader Southwest Airlines’ fares — an absence that also hurts the other aggregators and Internet travel agencies.