Mechanism for recording/reporting on automated lead behavior

I'm looking for the best way to record automated lead activity in Sell. By automated I mean web site actions including things such as downloading a brochure or white paper, requesting more information, filling out a contact form, things like that. I have tools that let me put information into Sell like lead source into the standard source field, create a new lead, update an existing lead. The important part of how I record this activity is being able to report on it later. A single lead might have several of these web actions I want to track or make decisions on as a group having performed the same web site action.

My concern is that if I put in the information (downloaded WhitePaper #3) as contents of a Note, I can't generate a report to show me all the leads who have performed the same activity. I can see it in the individual lead card, but not across all leads. Changing the Source field isn't an option because I still need to know how they first became a lead, not change it every time they fill out a different web form and displace the previous source. I could create a completed Task, but still couldn't create a report that only shows me tasks with the same label. I could tag a Note, but then I can only filter tags within a lead card instead of creating a report of all leads with notes having that tag. And the number of tags could get quickly out of hand. I don't want to use regular tags because that's already gotten out of control with the number of possible tags, and it litters the left side of the lead card to the point I don't even look at the tags anymore.

What are my options? What are other people doing?

Similarly I might have a source like "Webinar attendee" but where do I keep track of which webinars they attended? How can I pick all the leads who attended one specific webinar out of 50 possible webinars? How do I track the individual non-sales-rep actions without adding 50 tags?