I've been frustrated with "Event Calendars" forever. They either don't have enough events listed, are missing features, or not popular enough. I planned this to be a comprehensive look at all the major concert listing sites where I would rip each apart and then day dream about the features I wish somebody would wrap up into a single site. Up to this point, being unsatisfied with my choices, I keep a Backpack page with my list of concerts/events to share with my friends. Little did I know that one of these sites would answer practically all of my wishes.
I compared the recently launched Eventful, the recently acquired (by Y!) Upcoming, the unavoidable beast that is Ticketmaster, and two lesser knowns Pollstar and Mojam.

Criteria Defined
Prelisted Events
For an event calendar to be successful and really useful, it needs to be supported by the record labels and venues. This means it needs to be prelisted by these companies so that little Johnny Concertgoer doesn't have to fill them all in himself. Sure I don't mind filling in little random shows or house-parties but things like Weezer concerts should already be listed for me.
Add Events
While prelisted events are a neccessity, they just have to accept defeat right now and understand that it is impossible to get every tiny venue to list their events. Adding events is crucial in an event calendar so you can add the poetry reading that not many people might know about.
Event Alerts
Like most, I forget, and this is where alerts become critical. Everyone hates finding one of their fav bands played last weekend. Event Alerts means automatically finding out when a band, or a venue adds a show near you. RSS, iCal, and Email subscriptions are the three choices here (so far) and in an ideal world all three would be offered.
Tags or Categories
There are a million events but I only care about 10,000 of them. There has got to be a way for me to sift thru the rap and football to find indie films and rock music. Folksonomy (aka tags) are ideal over categories (i'll let you do your own research on why) but Categories have their place too, since, lets face it, events fit into a set of basic categories pretty naturally (music, plays, sports, etc).
Track Friends Events
This is where the social part comes in. Letting users keep track of events friends are attending actually make use of keeping all this 'itinerary' information online.
Developer API
Let your users expand and use your work in ways you didn't imagine (or were too lazy to program yourself).
Custom Calendars/Groups
Similar to "Adding & Tracking Friends" except in a wider reaching sense. What if none of my friends listen to punk-rock? Creating a custom calendar/group like "Punk Rock in Gainesville, FL" allows me to share my taste in events with a wider set than just people I know. Help people discover new bands in their fav genre, meet new people that actually do like the music you like, this is how that will happen.
Comments on Events
Allowing event entries to have comments also broadens the whole social aspect of an event listing. Sell your extra tickets, find out where the free parking is, ask if the opener is worth seeing.
Extra Extra Extras
The following are just dream world extra bits that would be nice on an event listing site. Links to maps of venues (so I don't have to google it myself), selling tickets on the same page (or atleast providing a link, if available), listing all kinds of events (and not just music, because lets face it, who wants to go to seperate sites to list their sporting and music events?), and just plain ol' attractive on the lookin' balls.
The Winner
The baby of the group, Eventful, was just launched September 22nd. Maybe its the last player advantage or maybe its the CEO's grasp on what web 2.0 is (and isn't). Whichever the case, I think I can easily declare Eventful the winner. It's what concert (or any event) social software aka C.S.S. ;-) should be. Its got folksonomies, true sharing and interactivity, radical trust (wikipedia style), and syndication in every way you could want (rss, iCal, email alerts, event boxes for your website).
I can actually only think of one complaint. I'd like Eventful to, at the very least, force me to pick atleast a broad category. A problem I had when searching for concerts is many events weren't tagged with concert (or music etc) so I had to sift through plays, sporting events, etc to find music concerts. Of course this would go away if everyone started tagging properly (including event promoters).
Edit: Originally I charted that Upcoming didn't provide an API. I was mistaken, but that doesn't change my outcome because prelisting events is SO important to having a one stop event listing venue. Perhaps less so for places like NYC and SF but I don't want to list every single Small Town USA event myself.
What now? Take Action!
For something like Eventful to work it needs to be embraced. Obviously this starts with you and your own events, but we really need to get record labels, promoters, venues etc to get behind it. Send them an email, link to this entry (if you want), just wake them up to what could be a beautiful thing for everyone. Hey it's free and could get some more people at their show.
Oh yeah, say good bye to finding out your favorite band played last weekend.
10:30 AM on 10.12.05
so that's why you asked where i find out about concerts online.