FIGES and Webring
Webring, a somewhat forgotten word today, of mostly historical significance, generally associated with the past tense. It used to mean a 'circle' of websites connected by a shared subject or some other loose affiliation. According to Wikipedia, they were popular in the 1990s and early 2000s, particularly among amateur websites. The circular structure means there are only peers, not much hierarchy or authority.
These are the languages globally active or globally marketing clients may be interested in, or just business who trade abroad. Depending on your circumstances, or those of your clients, Portuguese, Arabic or Chinese could be on the list of languages relevant to your clients, all of which — just like FIGES — exist in multiple variants. Also perhaps Russian, Hebrew, Polish, some major Indian or African languages. And everything local which pops up.
Apart from languages and directions (clients don't necessarily understand that not everyone is bidirectional) there must also be specialist areas you don't touch at all or when the text is too difficult.
While clients may understand that you can't be expected to know all languages, directionality may be more of a problem and specific disciplines even more, unless perhaps you advertise ('position') yourself as a legal or medical translator.
It's not even that your client would necessarily be disappointed with you if you didn't provide for a specific subset of that client's translation needs — stop thinking about yourself as a tiny agency! — or want to punish you for being too narrow, but if the client heads to an agency you risk losing all or some of the business.
The risk is there not only because of price competition from the bulk market but simply the comparative ease of ordering all languages and fields in one place versus the hassle of remembering about you with your specific individual languages and fields. To you, your freelance business is one of the central things in your life. To your client, not really. (It works both ways.)
You want to protect yourself from that eventuality, but you didn't become a freelance translator to manage projects or recruit for your clients. You would have started an agency for that. Even a partnership or joint venture of translators, and I've heard of two or three, would come very close, perhaps too close for your liking.
Still, if you have a physical office, and perhaps even if you don't, it would make sense to have a case handy with business cards from your colleagues. You could then give some pointers to a prospective client without officially and legally assuming responsibility for the relationship. While you aren't an agency, there could be added value in becoming a contact point and facilitator. It would make you harder to live without.
I mentioned webrings in the beginning because they're a cool thing that's no longer done, while they could serve a useful purpose for freelancers, similarly to a blog roll on a blog. Their circular structure, like a round table, is compatible with the freelance approach, and discreet, without restricting you too much. Thus, I think they're worth reinventing.
Plus, your colleagues will probably get more traffic from links on your website than they could from your word of mouth alone. And, if the information is provided right on your website, chances are your clients won't call or write to ask, which will help you focus on the work you do for them instead of being interrupted to talk about language or discipline you don't do.
Alternatively, depending on your specific circumstances, you could include lawyers, copywriters, designers, some brand of consultants, industry-relevant tradesmen, even doctors if you are locally based.
In addition, if you have already successfully co-operated with another freelancer, it may be a good idea to include that colleague in a case study on your website or in your portfolio. You can also invite people to write guest posts for your newsletter or blog and do the same for them.
A colleague who gets a new client because of you will likely feel the need to return the favour, and referrals are still the best source of work.