Related Links

Alternative solutions

Redirectors:

Apache2::Geo::Mirror Perl module to find closest Mirror written by Randy Kobes and T. J. Mather. In use by CPAN itself.
Matt Domsch's MirrorManager, as used by the Fedora Project [trac]
Mozilla.org's Bouncer
Raphael Geissert is working on an HTTP-based redirector for Debian. See https://github.com/rgeissert/http-redirector and http://http.debian.net/
Ryan Gordon's mod_offload
Mandriva Linux has a download redirector that works together with their urmpi client. I don't know where to find it.
The Apache Software Foundation uses a Perl script called closer.cgi to create their mirror lists
openSUSE/MirrorCache resembles MirrorBrain and attempts to improve on it

DNS-based solutions:

Apache Traffic Control - an Open Source CDN https://trafficcontrol.apache.org/
GeoDNS (geographically-aware DNS resolution), as used by e.g. http://kernel.org/. A little rigid, as it expects all servers to be tightly synced at all times.
The Coral Content Distribution Network is DNS-based, but not transparent to client applications.
DNS round-robin, as used by Debian: simple scheme, which is a solid approach for its purpose, but generally requires a client that is configured to access an appropriate server. Requires servers to be tightly synced at all times.

Another example for DNSrr is the Ring Server Project.

Proxy networks:

CoDeeN is a proxy server system created at Princeton University and deployed for general use on PlanetLab. To access this system, URLs are prefixed with http://coblitz.codeen.org/.
Cacheboy is a software in development to build a system of interconnected proxies. It is based on a Squid 2 derivative and, according to the documentation, requires root access on all mirrors.

Comments/additions welcome!

Some papers I found interesting.

A Taxonomy and Survey of Content Delivery Networks (2007). A detailed overview on existing commercial and academic CDNs. http://www.cloudbus.org/reports/CDN-Taxonomy.pdf
Tom Leighton from Akamai Technologies on Improving Performance on the Internet (2008): http://mags.acm.org/queue/200810/?pg=24
Towards an Accurate AS-Level Traceroute Tool (2003): http://www.routeviews.org/papers/AS_traceroute.pdf
Predicting Internet Network Distance with. Coordinates-Based Approaches (2002): http://www.cs.rice.edu/~eugeneng/papers/INFOCOM02.pdf
IDMaps: A Global Internet Host Distance Estimation Service (2001): http://idmaps.eecs.umich.edu/papers/ton01.pdf
Geographic Locality of IP Prefixes: http://www.coralcdn.org/docs/ipgeo-imc05.pdf
On the scale and performance of cooperative web proxy caching (1999): http://www-cse.ucsd.edu/classes/fa01/cse222/papers/wolman-coopcache-sosp99.pdf
An interesting and related research topic is the reasonability of P2P traffic with regard to network topology, called ''Application-Layer Traffic Optimization (ALTO)'' problem. See http://alto.tilab.com/. (See P2P in wikipedia for overview about swarm downloading technologies.)
IETF draft "Framework for CDN Interconnection" https://datatracker.ietf.org/doc/draft-ietf-cdni-framework/?include_text=1