Zotero
Zotero offers bibliographic references, a limited number of supported sites, notetaking capabilities, integration with WordPress and Microsoft Word. You can search, add notes, add tags and attachments.
The Zotero demo can be found at: http://www.zotero.org/documentation/screencasts/intro. In a world where it's all about customization, Zotero's header states that it is, "Leveraging the long tail of scholarship". Zotero is being coined as the "Firefox Scholar" or "SmartFox". It's free to download and although the source is not readily available, they claim that it is "open source". To that end, they do have an email contact for those interested in participating with the development efforts.
On a downside, as this is targetted towards the academic community, it is a bummer that they did not integrate with LaTeX. For as long as LaTeX has been around, it is still the program of choice for researchers today. It is to them what Word is to everyday business.
Onfolio for Windows Live Toolbar
Though not directly geared at academia, Microsoft has a comparable offering called Onfolio for Windows Live Toolbar. Onfolio, originally started by Coldfusion inventor JJ Allaire, was acquired by Microsoft last year. There is an advanced bookmarking feature and you can save partial or complete web sites. You can organize your research collections in an Explorer like experience and you can search this. There are also collaboration features.
Google Notebook
And then of course you have Google Notebook. You can't really categorize this in the same class as the two mentioned above, but is the closest offering that Google has to this niche. This is probably the weakest offering of the ones mentioned thus far. As the product title suggests, it is merely a notebook. You type notes into it and save. The features you ask? You can search for your notes in Google Notebook =). Ok, so that's not exactly revolutionary you say. Well, aside from interacting with Notebook via the browser, you can download it as a browser extension. Unfortunately, in the time that I spent with the extension, I became so annoyed with its lack of features that I uninstalled it altogether. Mind you, my google extension experience was a few months back, so it may be possible that their extension has improved significantly over this time. Although, having revisited their Notebook portal, it doesn't look promising.
Conclusion:
The best bet right now for feature rich research browser extensions are Zotero and Onfolio. It was difficult to compare each of them due to the different feature sets offered. However, one thing was consistent across all three tools. Their purpose was to organize your data and research from the internet. And all three of them do so with browser extensions. Zotero caters particularly well for the academic world. Onfolio caters well for businesses. And Notebook, just reminds me of the Post-it notes desktop tool, except it doesn't let me save on my desktop.