I thought when you have a consonant-stem verb ending with _-ku_ you replace it
with _-ita_. For example _ka **ku_** ("to write") becomes _ka **ita_**.

But this doesn't happen with _i **ku_** , which becomes _i **tta_** , so I
guess that makes it irregular – are there any other such verbs?
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verbs conjugations irregularities-exceptions
Are there more irregular verbs like 行く?
Yes, 行く/iku is an exception to the rule. Specifically, its -te/-ta form is
itte/itta. However, in most ways this verb is regular, so it usually does not
get included in the list of exceptions, which is only two verbs long: する/suru,
and 来る/kuru.

One other point of note is that 行く is sometimes pronounced "yuku," although
this is slightly less common.
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Teachers and intermediate language learners regularly tell beginners that
Japanese has only two irregular verbs (来る and する). This is not, strictly
speaking, true.

Here are some others:

  * 行く has the irregular forms 行った and 行って, as you've noted.
  * ある has the negative form ない... which is quite irregular.
  * Imperatives often seem to have irregular forms, most notably くれ from くれる.
  * Honorific verbs can be considered as having their own pattern, or breaking the rules (e.g., くださる⇒ください).
  * 問う ⇒ 問うた and 請う ⇒ 請うた, as were pointed out by @snailboat.
  * There are a few historical pronunciations that sometimes complicate this as well, such as the abbreviated する in 訳す or 達す. The 〜する irregularity also extends to the 〜じる variant in verbs like 禁じる.

More can be found on the ["Japanese irregular verbs" Wikipedia
page](http://en.wikipedia.org/wiki/Japanese_irregular_verbs).
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Here is how you derive the past tense for 行く (and the te-form, equivalently):

```

       past tense of /iku/  
    == { past tense of X = continuative form of X + /ta/ }  
       /iki/ + /ta/  
    == { generating a new word requires sound-changes }  
       repair(devoicing(/ikita/))  
    == { /i/ and /u/ become devoiced/disappear in-between two devoiced consonants }  
       repair(/ikta/)  
    == { normally, I-replacement repairs /k/+consonant; but here, gemination }  
       /itta/
    
```

Basically, it comes down to a devoiced vowel resulting in a consonant-
consonant sequence, and that `CC` sequence getting repaired somehow.

For all other -ku verbs, e.g., `/kaku/`, we get `/kakita/ => /kakta/ =>
/kaita/` , using I-replacement.

So the exception with `/iku/` is that gemination repairs the `/k/+C` sequence
instead of I-replacement.

That is the only exception I know which pertains to repairs, to directly
answer your question (instead of the question "what words conjugate weird?").

* * *

As to why this happens with 行く, I am not sure... in 徒然草 (1330-1332),

> 用有りて **行きたり** とも、其の事 果てなば、とく歸るべし

and even much more recently, in 不如帰 (~1898)

> 今まで知らぬ自由と楽しさのこのさきに待つとし思えば、父に別るる哀しさもいささか慰めらるる心地して、いそいそとして **行きたる** なり。

we indeed have proof it was, at least in writing, the continuative form plus
the past(/perfect) morpheme, with no repairs. At some point, this changes, see
こゝろ (1914):

> 私は早速先生のうちへ金を返しに **行つた** 。

so this doesn't really help us figure out anything very much at all. I can't
seem to track down anything useful here.

* * *

I'd classify 問う and 請う as a different "exception" from 行く, because for
whatever reason, a totally different sound change happened, from -ひた- to -ふた-.
We can still see the older form here,

> 女に思ふやと **問ひたり** けれどいらへもせざりければ
>
> 思ふともいはずなりぬる時よりも増る方にて頼まるゝ哉

from 左近大將朝光 in the 後拾遺和歌集 (~1086). This [-うた
form](https://japanese.stackexchange.com/questions/3299/mukashi-banashi-do-
they-borrow-from-other-current-dialects-in-addition-to-older/3312#3312) is
still around in the Osaka dialect and is used for the past tense and te-form
regularly. It's not clear to me why there was this branch in pronunciation.
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