At the Internet Archive we've created Heritrix for 'archival quality' crawling -- especially when you want to get every media type, and sites to complete/arbitrary depth, in large but polite crawls. (It's possible, but not usual for us, to configure it to only collect textual content.)
The Nutch crawler is also reasonable for broad survey crawls. HTTrack is also reasonable for 'mirroring' large groups of sites to a filesystem directory tree.
(1) Add a scope rule that throws out discovered URIs with popular non-textual extensions (.gif, .jpe?g, .mp3, etc.) before they are even queued.
(2) Add a 'mid-fetch' rule to FetchHTTP module that early-cancels any fetches with unwanted MIME types. (These rules run after HTTP headers are available.)
(3) add a processor rule to whatever is writing your content to disk (usually ARCWriterProcessor) that skips writing results (such as the early-cancelled non-textual results above) of unwanted MIME types.
'Broad' crawl means roughly, "I'm interested in the whole web, let the crawler follow links everywhere." Even starting with a small seed set, such crawls quickly expand to touch hundreds of thousands or millions of sites.
Even as a fairly large operation, you're might to be happy with a representative/well-linked set of 10 million, 100 million, 1 billion, etc. URLs -- which is only a subset of the whole web, hence a 'survey'.
A constrasting kind of crawl would be to focus on some smaller set of sites/domains you want to crawl as deeply and completely as possible. You might invest weeks or many months to get these deeply in a gradual, polite manner.
The Nutch crawler is also reasonable for broad survey crawls. HTTrack is also reasonable for 'mirroring' large groups of sites to a filesystem directory tree.