- A report that gives the solution to a problem. Not just any problem; but a very annoying, frustrating problem where their life will be changed if they can apply the solution.
- An ebook that explains step-by-step how to do something. What do readers in your niche most want to know how to do? That is what will make for a good How To ebook.
- A free tool that helps them do something easier, better, or faster. Who isn't interested in making their lives easier? The more frustrating the problem is, the more likely they'll want the tool.
- Something that they can turn around and give away too to build their lists. This also falls under the "make my life easier" heading - if they can get their hands on a report they can give away too, then they'll see that as a desirable report to grab.
- An audio recording of an interview with an expert in your field who is sharing new information. Experts know a lot, but what's going to draw the most opt-ins is the expert's new info and/or top secrets.
Keep in mind it's possible to have more than one freebie, and even desirable in many cases. Try one with a traditional opt-in box above the fold on your site, then perhaps another box for something else on a different page of your site where the opt-in complements the post there. Try a third in the sidebar linked to a banner ad. Check your results to see which performs the best.
Building your list is an ongoing process and one vital to your success online. Ask your readers what they most want based on the suggestions above and give it to them. It will not only increase your subscriber numbers, but eventually your income.