Forewarning: This question is highly subjective and can easily be answered with "Add a bookmark you potato" but I wanted to point out a scenario which I come across all too often.
I frequent Stack Overflow and love the "Hot Network Questions" list on the right-hand side so I often click on questions of interest and have been inclined on several occasions to join a community just because I felt that I can provide a solution or comment.
Some Stack Exchange sites are frequented more than others and I will often find myself wanting to visit, for example, the User Experience one.
It would seem intuitive to type http://userexperience.stackexchange.com but alas it is wrong.
If I want to visit Arqade then I must use http://gaming.stackexchange.com
Fraught with peril, I wish to visit the Graphic Design one so instead I run a Google search just to find that I could have trusted my instincts because it is in fact http://graphicdesign.stackexchange.com
I am thoroughly aware of http://stackexchange.com/sites but the listing is formed based on popularity of the site and most blocks are small with barely distinguishable logos.
I am also aware that I can see all of the communities which I have joined by visiting my profile but it is also prone to the issue of too many choices and it actually requires me to read something.
I think it would be beneficial to users and even the StackExchange network if an autocomplete search field could be implemented which could match keywords to the various networks.
Once again, this is clearly subjective and I would love to hear any Stack Exchange experts weigh in on this.