Gmail is a great tool. More than 100 million people use it, including me. Google (Nasdaq: GOOG) knows this, of course, and in its efforts to grab share from market leaders Yahoo! (Nasdaq: YHOO) and Facebook, the Big G has taken to inserting display ads into Gmail windows.

"We're always trying out new ad formats and placements in Gmail, and we recently started experimenting with image ads on messages with heavy image content," Google said in a statement to Search Engine Land, which broke the news.

As a Google shareholder, I'm interested in taking a closer look at this latest profit play. Unfortunately, I can't. I've been shut out, and it's entirely my own fault.

How so? I'm an advanced Gmail user. I've tricked out my Mac's Chrome browser with a series of plug-ins, including the Gist add-on that dominates the right side of the screen in my Gmail window. Ads that might otherwise go there are overwhelmed by Gist's code.

Will having a rich market for plug-ins and extensions ruin Google's chances to gain on Yahoo! and Facebook and separate itself from laggard AOL (NYSE: AOL) in display advertising? That's probably going too far. But we also know right-side display ads work.

Take Facebook. Research firm eMarketer says the social superstar is on track to book more than $4 billion in global revenue in 2011. And in August, Chief Operating Officer Sheryl Sandberg let slip that some of the site's biggest advertisers had increased spending 10-fold.

Gmail has yet to prove it has the chops to attract this sort of outsized interest from display advertisers. But with plug-ins getting in the way, it may also never get the chance. Should investors care? Let us know what you think using the comments box below. You can also rate Google in CAPS.

Interested in more info on the stocks mentioned in this story? Add AOL, Google or Yahoo! to your watchlist.

Both our Motley Fool Inside Value and Motley Fool Rule Breakers services have recommended members purchase shares of Google. Yahoo! is a Motley Fool Global Gains pick. You can also try any of our Foolish newsletter services free for 30 days.

Fool contributor Tim Beyers is a member of the Rule Breakers stock-picking team. He owned shares of Google at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. You can also get his insights delivered directly to your RSS reader. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool owns shares of Google and is also on Twitter as @TheMotleyFool. Its disclosure policy must go eat dinner now. Bye.