Start-ups and big companies alike have tried to take on Google by building a better search engine. That they have failed has not stopped brave new entrants.

The latest is Blekko, a search engine that will open to the public on Monday.

Rich Skrenta, Blekko’s co-founder and chief executive, says that since Google started, the Web has been overrun by unhelpful sites full of links and keywords that push them to the top of Google’s search results but offer little relevant information. Blekko aims to show search results from only useful, trustworthy sites.

“The goal is to clean up Web search and get all the spam out of it,” Mr. Skrenta said.

Blekko’s search engine scours three billion Web pages that it considers worthwhile, but it shows only the top results on any given topic. It calls its edited lists of Web sites slashtags. The engine also tries to weed out Web pages created by so-called content farms like Demand Media that determine popular Web search topics and then hire people at low pay to write articles on those topics for sites like eHow.com.

It is also drawing on a fruitful category of Web search — vertical search engines that offer results on specific topics. Many companies assume that Google won the contest to search the entire Web, so they have focused on topical search. Bing from Microsoft has search pages dedicated to travel and entertainment, and Yelp is a popular choice for searching local businesses.