Download the new RouterOS 6.0beta1 release over the torrent network: http://www.mikrotik.com/download/routeros-ALL-6.0beta1.torrent
Otherwise please visit http://www.mikrotik.com/download to pick exact version!
6.0beta1 changelog:
*) updated drivers and kernel (to linux-2.6.38.2);
*) improved interface management (scales well for up to thousands of interfaces and more);
*) improved queue management (/queue simple and /queue tree) - easily handles tens of thousands of queues;
*) improved overall router performance when simple queues are used - at least double the performance of v5, even bigger improvements on multicore systems;
*) very small overhead for packets that miss simple queues, but simple queues are present in the system;
*) pcq queue is NAT aware (just like "/queue simple" and "/ip traffic-flow";
*) in "/ip firewall mangle" can specify "new-priority=from-dscp-high-3-bits";
*) new default queue types: pcq-download-default and pcq-upload-default;
*) simple queues have separate priority setting for download/upload/total;
*) slave flag shows up for interfaces that are in bridge,bonding or switch group;
*) global-in, global-out, global-total parent in /queue tree is replaced with global that is equivalent to global-total in v5;
*) simple queues happen in different place - at the very end of postrouting and local-in chains;
*) simple queues target-addresses and interface parameters are joined into one target parameter, now supports multiple interfaces match for one queue;
*) simple queues dst-address parameter is changed to dst and now supports destination interface matching;
*) dns cache logs requests to topics "dns" and "packet";