<p>The software base I am developing for uses a signficant amount of yacc which I don't need to deal with. Some times I think it would be helpful in understanding some problems I find but most of the time I can get away with my complete ignorance of yacc.</p>

<p>My question are there enough new projects out there that still use yacc to warrant the time I'll need to learn it?</p>

<p>Edit: Given the response is mostly in favour of learning Yacc, is there a similar language that you would recommend over yacc?</p>

Yes, these tools are worth learning if you ever need to create or modify code that parses a grammar.
For many years the de facto tool for generating code to parse a grammar was yacc, or its GNU cousin, bison.
Lately I've heard there are a couple of new kids on the block, but the principle is the same:  you write a declarative grammar in a format that is more or less in Backus-Naur Form (
BNF
) and yacc/bison/whatever generates some code for you that would be extremely tedious to write by hand.
Also, the principles behind grammars can be very useful to learn even if you don't need to work on such code directly.  I haven't worked with parsers much since taking a course on Compiler Design in college, but understanding runtime stacks, lookahead parsers, expression evaluation, and a lot of other related things has helped me immensely to write and debug my code effectively.
edit:
Given your followup question about other tools, Yacc/Bison of course are best for C/C++ projects, since they generate C code.  There are similar tools for other languages.  Not all grammars are equivalent, and some parser generators can only grok grammars of a certain complexity.  So you might need to find a tool that can parse your grammar.  See
http://en.wikipedia.org/wiki/Comparison_of_parser_generators