It's always difficult to show good results for general terms, that's why search engines suggest refinements to precisely define what you want to find. Yahoo displays them both at the top and at the bottom of the page, while Ask has an entire sidebar for search suggestions. Clusty aggregates results from other search engines and clusters them dynamically.
Google was more conservative and placed the related searches at the bottom of the page and sometimes in the middle of the page as an "inline revision", but now it started to show the queries at the top of the page. Usually the related searches add one or two words that disambiguate the query. They're also useful as a guidance if you don't know too much about a certain domain and you want to explore it.
From all the search engines, Ask.com is the most courageous because it offers a wide variety of suggestions while typing your query, so it can drastically improve the quality of your search query. In many cases, all you need to obtain great search results is a well-chosen query and these suggestions, obtained from other users that manually refine the queries, are helpful.