We have already discussed the basics of site syndication. We have also mentioned that multiple feed formats make publishing syndicated content problematic.
Today we are going to focus on some free online services that help merge multiple feeds into one. Typically these services not only output resulting feed in unified feed format but they also provide an easy way to publish this feed on your site.

If we are to publish a link to another sites' article we will generally need these elements:

• Title
• Summary
• Publish date
• Author
  
• Link to the article
• Source
• Link to the source

Luckily most of these elements are present in all feed format specifications (and all of them - in RSS 2.0).

We are going to compare the services paying specific attention to the handling of RSS. As our source we will use RSS 2.0 feed from "A List Apart". We chose it because this feed belongs to the site that advocates compliance to web standards and as such it obviously follows specs to the letter.

FeedDigest.com

FeedDigest converts feeds to RSS 2.0, Atom, WAP and HTML. HTML is exported using editable templates. The system checks if the source feed is well formatted and will not add it otherwise. All the elements we need are present except for "Author".

Verdict:

Very good. Absence of "author" element is noticable, but not critical.

RSSMix.com

RSS Mix allows you to mix any number of RSS feeds into one unique new feed. No login necessary. Since there is no login there is no way to edit your mix. Resulting feed is RSS 0.92. The only elements present are "Title", "Summary", "Publish date" and "Link to the article".

Verdict:

Not worth it. Older format and lack of flexibility.

RSSMix.ru

This site is in Russian.

RSSMix.ru allows you to mix feeds into one RSS 2.0 feed and/or get them via email. As in our previous candidate only the very basic elements are available. The difference is a "Link to the article" element. The link does not lead to the actual article, but rather to redirect page on RSSMix.ru. This is done to provide you with usage statistics for your mix. I am guessing that many sites that syndicate their content will not be happy with this. One of the reasons for content syndication is boosting search engine rankings through reciprocal links to the source site. The users, on the other hand wont feel a difference.

Verdict:

Not worth it. Limited amount of elements available, no direct links.

If you know of any other services that allow feed mixing and conversion post a comment and I will try to review them too.